Question: Solve for $r$ : $13 + r = 8$
Solution: Subtract $13$ from both sides: $ \begin{eqnarray} \\ 13 + r &=& 8 \\ \\ {-13} && {-13} \end{eqnarray} $ $ \begin{eqnarray} \hphantom{13 + r} &\hphantom{=}& \hphantom{8} \\ r &=& 8 {- 13} \\ \end{eqnarray} $ Simplify. $ r = -5$